1. Field of the Invention
The present invention relates to a laser oscillation apparatus for generating laser light by oscillation and optical amplification by means of a pair of optical amplification mirrors. In particular, the present invention relates to a laser oscillation apparatus improved with respect to at least one of a high voltage power source circuit for generating a discharge, resulting in an enhanced freedom in design, a control unit for a cooling mechanism which allows a stable laser output to be achieved in a short period of time after start-up in a cold atmosphere, and a laser light absorption unit for receiving and absorbing laser light and exchanging heat with a coolant.
2. Description of the Related Art
The entire disclosure of U.S. patent application Ser. No. 08/885,101 filed Jun. 30, 1997 is expressly incorporated by reference herein.
FIG. 14 is a diagram schematically illustrating a configuration around a laser cavity unit 1100 in a conventional laser oscillation apparatus.
In the laser oscillation apparatus shown in FIG. 14, a laser cavity unit 1100 includes a laser tube 106, a partially-transmissive reflection mirror 104, and a total reflection mirror 105. A high voltage is applied from a DC high voltage power source 102 via discharge electrodes 103a and 103b to a gaseous laser medium 101 contained in the laser tube 106 so as to generate a glow discharge. A blower 107 and a laser medium cooler 108 are serially connected to the laser tube 106 via laser medium conduits 109a and 109b. The laser medium 101 is forcibly circulated by the blower 107. Particularly, the gaseous laser medium 101, heated by the glow discharge, passes through the laser medium conduit 109b, is cooled by the laser medium cooler 108, passes through the blower 107 and the laser medium conduit 109a, and then is sent back to a glow discharge space in the laser tube 106.
The total reflection mirror 105 is provided at one end of the laser tube 106, and the partially-transmissive reflection mirror 104 is provided at the other end thereof. Laser light generated by a discharge passes through the partially-transmissive reflection mirror 104 and exits the laser tube 106.
In the laser oscillation apparatus shown in FIG. 14, the DC high voltage power source 102 is directly connected to the discharge electrodes 103a and 103b via feeder cables 111a and 111b. Furthermore, a cathode of the DC high voltage power source 102, which is connected to the discharge electrode 103b, is grounded by the grounding conductor 110.
In the conventional laser oscillation apparatus having such a configuration as described above, during operation for producing laser light, a DC high voltage E (V), which corresponds to the supplied voltage level of the DC high voltage power source 102 (with the ground level being the reference level), appears at the discharge electrode 103a. (In this application, voltage that is expressed using the ground level as the reference level is referred to as "voltage to ground".) In such a case, the feeder cable 111a must have a sufficient anti-breakdown property so that it can withstand the DC high voltage E (V). The need for a feeder cable with such a high anti-breakdown property disadvantageously increases cost for conventional laser oscillation apparatuses.
Moreover, since the DC high voltage E (V) appears at the discharge electrode 103a, it is necessary to provide components constituting the laser oscillation apparatus around the discharge electrode 103a (e.g., a casing body) so as to be disposed with a sufficient distance therebetween depending on the voltage level of E(V) in order to prevent a discharge from being generated between the discharge electrode 103a and the surrounding other components. As a result, design of a laser oscillation apparatus is limited, and further, miniaturization of a laser oscillation apparatus becomes difficult.
Next, a cooling mechanism for optical components included in a conventional laser oscillation apparatus will be described with reference to FIGS. 15 and 16.
FIG. 15 is a diagram schematically illustrating an exemplary configuration of a cooling mechanism which can be used by being connected to the laser cavity unit 1100 of the laser oscillation apparatus described above. Elements in FIG. 15 which are also shown in FIG. 14 are denoted by the same reference numerals and will not be further described.
In the configuration shown in FIG. 15, optical components such as the partially-transmissive reflection mirror 104 and the total reflection mirror 105 are held by a holder 207. During operation of the laser oscillation apparatus, some thermal energy from a discharge may be applied to the holder 207, and thus, the holder 207 may be deformed by thermal expansion, resulting in deteriorated positional parallel relationship between the partially-transmissive reflection mirror 104 and the total reflection mirror 105. Similarly, when the temperature of the holder 207 is considerably decreased, the partially-transmissive reflection mirror 104 and the total reflection mirror 105 may be shifted with respect to each other from the predetermined positional parallel relationship due to contraction of the holder 207 induced by low temperature. This shift also leads to the deteriorated positional parallel relationship. If the partially-transmissive reflection mirror 104 and the total reflection mirror 105 are not disposed in parallel to each other, sufficient light amplification therebetween is not provided, in which case a stable laser light oscillation may not easily be achieved.
In order to overcome such a problem, oil, for example, is circulated within the holder 207 by means of a pump 208 to cool the holder 207. In particular, such a cooling mechanism using oil includes a tank 211, the pump 208 for supplying the oil into the holder 207, a cooler 210 for cooling the oil, and a thermistor 209 for detecting the oil temperature. Moreover, a control unit 212 is provided for controlling the operation of the cooler 210 based on the oil temperature detected by the thermistor 209. After the operation of the laser oscillation apparatus is initiated, the oil is cooled by controlling the operation of the cooler 210 according to a control loop as shown in a dashed line in FIG. 15.
FIG. 16 shows diagrams provided for illustrating problems associated with such a cooling mechanism for optical components in the conventional laser oscillation apparatus.
Particularly, the portion (a) of FIG. 16 schematically illustrates the change in the temperature of the oil in the cooling mechanism from shutdown to some time after subsequent start-up. The temperature indicated therein can be considered as the temperature of the holder 207, which is cooled by the oil. Moreover, the portion (d) of FIG. 16 is a diagram schematically illustrating the change in the laser output of the laser oscillation apparatus after start-up, and the portions (b) and (c) of FIG. 16 illustrate the operation timing of the pump 208 and the cooler 210, respectively, after start-up.
When the conventional laser oscillation apparatus is standing in a cold atmosphere, for example, in winter, the temperature of the holder 207 becomes considerably lower than the normal operating point temperature of the laser oscillation apparatus. Accordingly, the oil temperature becomes also low as shown in the portion (a) of FIG. 16. Due to such a considerably low temperature, a great amount of time may be required for warm up of the holder 207 to an operating temperature, which is shown as the oil temperature change in the portion (a) of FIG. 16, after the oscillation apparatus has started its operation at the time shown in the portion (d) of FIG. 16 and the pump 208 has accordingly started its operation at the time shown in the portion (b) of FIG. 16. Thus, the positional parallel relationship between the partially-transmissive reflection mirror 104 and the total reflection mirror 105 is shifted for a while after start-up, during which a stable light amplification (laser oscillation) can not be achieved, resulting in a lowered laser output. As a result, as shown in the portion (d) of FIG. 16, a great amount of time is required until the laser output becomes stable again at the normal operating level.
Once the laser output becomes stable at the normal operating level, the control unit 212 acts to cause the cooler 210 to operate at an appropriate time as shown in the portion (c) of FIG. 16. This allows for a stable operation of the laser oscillation apparatus.
Next, a laser light absorption unit included in the conventional laser oscillation apparatus will be described with reference to FIGS. 17 to 19.
The laser light absorption unit is provided on the optical path of the generated laser light. Normally, the laser light absorption unit is located so as to block the optical path of the laser light, thereby preventing the laser light generated in the laser cavity unit from exiting the laser oscillation apparatus at any time other than a desired time, thus functioning as a safety apparatus. Then, once it is confirmed that the laser light may exit (e.g., in a manufacturing site, when it is confirmed that the laser light has been aimed to an object to be processed and that there is no obstruction in the intervening path), the laser light absorption unit is shifted aside the optical path of the laser light so that the laser light exits the laser oscillation apparatus.
FIG. 17 is a cross-sectional view schematically illustrating a configuration of a conventional laser light absorption unit 1310.
In the laser light absorption unit 1310, a conically-shaped inner cylinder 301 is provided at an opening of an outer cylinder 304. The conically-shaped inner cylinder 301 includes a light-receiving surface 302 and a heat-exchanging surface 303 respectively provided on the front surface and the rear surface of the inner cylinder 301. A space existing between the conically-shaped inner cylinder 301 and the outer cylinder 304 provides a path 305 for a coolant 307. The conically-shaped inner cylinder 301 is formed of a metallic material having a high thermal conductivity, e.g., copper or aluminum.
The light-receiving surface 302 is formed in a conical shape with an angle of about 30.degree. or less with respect to the incident axis of the laser light 306 so that the incident laser light 306 is not directed externally after being reflected. Moreover, the light-receiving surface 302 is coated with a material having a high absorptivity for the wavelength of the laser light 306 to be oscillated.
The laser light 306 incident upon the light-receiving surface 302 is quickly absorbed, and the heat produced by the incident laser light 306 is transferred by conduction to the heat-exchanging surface 303. The coolant 307 introduced into the path 305 through an inlet 308 exchanges heat at the heat-exchanging surface 303 and is drained through an outlet 309.
FIGS. 18 and 19 are cross-sectional views schematically illustrating configurations of other conventional light absorption units 1320 and 1330, respectively. Elements in FIGS. 18 and 19 which are also shown in FIG. 17 are denoted by the same reference numerals and will not be further described.
In the laser light absorption unit 1310 shown in FIG. 17, the light-receiving surface 302 is formed in a single conical shape. This necessarily causes the light-receiving surface 302 to be large with respect to the incident axis of the laser light 306. On the other hand, in each of the light absorption units 1320 and 1330 shown in FIGS. 18 and 19, respectively, the light-receiving surface 302 is shaped so as to form a plurality of conical shapes, thus reducing the overall size. Also in these cases, the light-receiving surface 302 forms an angle of about 30.degree. or less with respect to the incident axis of the laser light 306.
Generally, laser light has the greatest energy concentration near the center thereof, while the energy concentration becomes smaller toward the peripheral portion of the laser light. Therefore, the light-receiving surface 302 in each of the laser light absorption units 1310 to 1330 must receive and absorb the greatest energy at the center thereof. The energy absorbed at the light-receiving surface 302 is transferred to the heat-exchanging surface 303 on the rear surface while substantially maintaining the temperature distribution thereof. Thus, the temperature on the heat-exchanging surface 303 also becomes highest at the center thereof, while the temperature becomes less toward the peripheral portion thereof. Accordingly, there are large differences in temperature along the radius direction on the light-receiving surface 302 and the heat-exchanging surface 303.
However, in the conventional laser light absorption units 1310 to 1330, the coolant 307 flows irrespective of the temperature distribution in the heat-exchanging surface 303. Therefore, the amount of the coolant 307 to be supplied in the vicinity of the center of the heat-exchanging surface 303, where the temperature is high, is not sufficient (i.e., the flow of the coolant 307 is insufficient). On the other hand, the amount of the coolant 307 to be supplied in the peripheral portion of the heat-exchanging surface 303, where the temperature is low, tends to be excessive. As a result, the heat exchange as a whole becomes non-uniform. Therefore, the temperature increases due to the insufficient cooling capacity near the center of the heat-exchanging surface 303, i.e., near the center of the light-receiving surface 302. This may result in considerable damage, and it would be difficult to maintain a sufficient quality of the laser light absorption units 1310 to 1330 over a long time.
Furthermore, the temperature of the coolant 307 after the heat exchange near the central portion of the heat-exchanging surface 303 becomes extraordinarily high. In some cases, the coolant 307 boils, whereby some vibration is generated. Such vibration may cause some mechanical damage to the laser light absorption units 1310 to 1330 and may hinder the laser oscillation apparatus from operating stably.